This invention relates to a pneumatic tire, and more particularly to a radial tire of so-called run flat type capable of running over a given distance at a state that an internal pressure becomes zero or is rendered into a slight pressure due to puncture or the like. Especially, it relates to a pneumatic tire having an excellent durability of run flat state (running at puncture state) for use in passenger cars, light trucks and small size trucks wherein a ratio of section height to section width (aspect ratio) is as relatively large as not less than 60.
Radial tires of run flat type (hereinafter referred to as a run flat tire) are mainly used in vehicles wherein a load applied to the tire is relatively small such as passenger cars, light trucks, small size trucks and the like. In the run flat tire, it is required that when the tire is rendered into a flat state through puncture during not only the running on general-purpose roads but also the high-speed running on express ways, it is possible to safely and surely run the tire up to a site capable of conducting tire exchange over a given distance, for example, not less than 300 km without damaging the steering stability of the vehicle, particularly passenger car and causing the separation off from a rim or the breakage of the tire.
For this end, run flat tires having various structures are proposed sometimes in a combination with a fully worked-out rim to be used. These proposed tires are roughly divided into super low section profile tires having an aspect ratio of less than 60 and tires having a relatively high section height corresponding to an aspect ratio of not less than 60.
The run flat tires mostly and practically used in markets among the above super low section profile tires are disclosed, for example, in JP-B-45-40483, JP-B-50-12921, JP-A-49, 70303, JP-A-49-116702, JP-A-50-59902, JP-A-50-60905, JP-A-50-60906, JP-A-50-60907, JP-A-50-78003, JP-A-50-111704, JP-A-50-121902, JP-A-50-138502, JP-A-51-20301, JP-A-51-64203 and JP-A-51-69804.
Giving an example of the tires disclosed in the above publications, as shown in FIG. 7, a tire 20 has a structure that a pair of thick reinforcing rubber layers 9 having a crescent shape at section are arranged at an inner face side of an innermost carcass ply 6-1 in a radial carcass 6 extending from a bead portion 2 through a sidewall portion 3 to a tread portion 4. However, this type of the tire is unavoidable in a point that the cost becomes higher and is frequently mounted onto an expensive grade of a vehicle supposing the high-speed running such as sport car, sport-type car or the like.
In the tire 20 having the thick reinforcing rubber layer 9, in order to mitigate the degree of crushed deformation during the running at run flat state under loading (hereinafter referred to as run flat running or running under low internal pressure) as far as possible, the radial carcass 6 has a two or more ply structure comprising a turnup ply 6-1 wound around a bead core 5 from an inside of the tire toward an outside thereof and a down ply 6-2 enveloping the turnup ply 6-1 from an outside thereof, and a hard stiffener rubber 8 extending from an outer peripheral face of the bead core 5 near to a position of a tire maximum width is disposed between the turnup ply 6-1 and the down ply 6-2, and a rubberized Kevlar cord or steel cord layer (a layer called as an insert ply) is arranged so as to extend from the bead portion 2 to the sidewall portion 3, if necessary.
On the other hand, the tires having the relatively high section height are frequently used in not only high-grade imported passenger cars and high-grade domestic passenger cars having a relatively large displacement but also light trucks and small size trucks. This type of the tire is possible to run at run flat state in cooperation with a rim fitted thereto, wherein a type of incorporating a protection member (core) for pushing the bead portion toward a rim flange into the rim is the main current as disclosed in JP-U-56-143102, JP-Y-4-11842, JP-U-2-64405, JP-U-2-64406, JP-A-5-104915, JP-A-6-48125 and JP-A-6-270617.
In the tire having the structure shown in FIG. 7, it is proposed and practiced to increase the thickness or height of the stiffener rubber 8 and the reinforcing rubber layer 9, or largely increase the hardness and modulus of rubbers 8, 9 themselves for improving the durability in the run flat running. These improving means are proposed under an intention that region xcex1 in the vicinity of the bead portion 2 and region xcex2 of the sidewall portion 3 being trouble region in the run flat running as shown in FIG. 8 are reinforced and at the same time the reinforcing balance of both regions xcex1, xcex2 is optimized to reduce strain at the region xcex1 and strain at the region xcex2 together.
However, as the reinforcing degree of each of the stiffener rubber 8 and the reinforcing rubber layer 9 increases, the occurrence of troubles in the region xcex1 and the occurrence of troubles in the region xcex2 as a tire trouble in the run flat running can be avoided, while the trouble site merely and rapidly shifts to a region xcex3 of a shoulder portion including an end portion of a belt 7 and an end portion of the reinforcing rubber layer 9 as shown in FIG. 7. Finally, the thus improved conventional run flat tire can not attain the run flat runnable distance of not less than 300 km and support the high-speed running on the expressway as user""s demand. Therefore, it is still desired to develop tires having a run flat durability capable of running over not less than 300 km and supporting the high-speed running on the expressway.
And also, the means of incorporating the core into the rim has a problem firstly in the rim assembling property because the assembling of the tire onto a wheel is not easy. Further, the big rise of weight in the tire and in the tire-wheel assembly is unavoidable and also unspring weight of the vehicle is largely increased to considerably damage the ride comfort of the vehicle against vibrations. That is, there is a problem that these inconveniences become unsuitable in not only the high grade passenger cars but also the light tuck and small-size truck.
It is, therefore, an object of the invention to provide a pneumatic tire particularly having an aspect ratio of not less than 60 which can hold a good rim assembling property without causing weight increase in tire-rim assembly and remarkable cost rise, guarantee the safe high-speed running of a vehicle such as passenger car, light truck, small size truck or the like even if rapid air leakage is caused by puncture or the like, and develop the performance of preventing separation off of the tire from the rim during the run flat running over not less than 300 km and the durability performance.
It is another object of the invention to provide a pneumatic tire having an aspect ratio of not less than 60 and an excellent ride comfort against vibrations during the run flat running, particularly a run flat tire for passenger car.
In order to achieve the above objects, the invention lies in a pneumatic tire comprising a carcass of one or more rubberized plies of radially arranged cords toroidally extending between a pair of bead cores embedded in respective bead portions and reinforcing a pair of sidewall portions and a tread portion, a belt of two or more steel cord cross layers disposed on an outer periphery of the carcass to reinforce the tread portion, and a reinforcing rubber layer arranged in at least a part of a zone ranging from a position near to the bead portion through the sidewall portion to a shoulder region of the tread portion, characterized in that
the belt has end portions at both shoulder regions of the tread portion;
at least one cushion rubber layer is provided between the end portion of the belt and the reinforcing rubber layer in the shoulder region and between mutually adjacent tire constitutional members; and
the cushion rubber layer has a loss tangent lower than a loss tangent (tan xcex4) of rubber for coating cords of the carcass ply.
The term xe2x80x9cshoulder region of tread portionxe2x80x9d used herein is defined by a region enclosed between a vertical line drawn to an inner face of an innermost carcass ply through a ground contact end and a vertical line drawn to the inner face of the innermost carcass ply through a position separated by xe2x85x9 width from the ground contact end toward a central side of a ground contact area when a ground contact width of the tread portion is divided into eight equal parts at a section of the tire when a tire-rim assembly formed by assembling the tire onto an approved rim is inflated under a slight pressure corresponding to 10% of a maximum air pressure (defined according to JATMA, TRA or ETORTO standard of 1998). Moreover, when the tire has a round shoulder, the ground contact end is an intersect between extension lines of two curved lines or between extension lines of one curved line and one straight line connecting to each end of an arc forming the round.
And also, the term xe2x80x9closs tangent (tan xcex4)xe2x80x9d used herein is a value measured at a testing temperature of 25xc2x0 C. according to (1) xe2x80x9cCase through loading waveform, deflection waveformxe2x80x9d among non-resonant methods described in xe2x80x9cTesting methods for dynamic properties of cured rubberxe2x80x9d of JIS K 6394-1995 when a kind of deformation is tension.
In a preferable embodiment of the invention, the cushion rubber layer is arranged between mutually adjacent end portions of the two steel cord cross layers constituting the belt, between the outermost carcass ply and an end portion of a steel cord layer nearest to the ply, between adjacent carcass plies in the carcass comprised of two or more plies, or between the innermost carcass ply and the reinforcing rubber layer.
In case of being arranged between the end portions of the steel cord cross layers, the cushion rubber layer has a width within a range of 10xcx9c30 mm and is favorable to dividedly arrange the cushion rubber layer at an equal width on both sides with respect to a vertical line VL1 drawn to the outermost carcass ply through an end of a narrow-width steel cord layer. This is a case that the mutually adjacent two steel cord cross layers have different widths. If the widths of the cross layers are the same, the vertical line VL1 may be a vertical line passing through any end of the steel cord layers.
In this case, it is favorable that a distance d1 between adjacent steel cords at the ends of the steel cord cross layers as measured on the vertical line VL1 is within a range of 0.5xcx9c2.0 mm through the cushion rubber layer.
In case of being arranged between the outermost carcass ply and the end portion of the steel cord layer, the cushion rubber layer has a width of 10=40 mm and is favorable to dividedly arrange the cushion rubber layer at an equal width on both sides with respect to a vertical line VL2 drawn to the inner face of the innermost carcass ply through an end of an innermost steel cord layer constituting the belt.
In this case, it is favorable that a distance d2 between cord of the outermost carcass ply and steel cord of the belt layer nearest to such a ply as measured on the vertical line VL2 is within a range of 0.5xcx9c6.0 mm though the cushion rubber layer.
In case of being arranged between the carcass plies, the cushion rubber layer has a width of 10xcx9c30 mm and is favorable to dividedly arrange the cushion rubber layer at an equal width on both sides with respect to the vertical line VL2.
In this case, it is favorable that a distance d3 between cords of the adjacent carcass plies as measured on the vertical line VL2 is within a range of 0.5xcx9c2.0 mm through the cushion rubber layer.
In case of being arranged between the innermost carcass ply and the reinforcing rubber layer, the cushion rubber layer has a width of 10xcx9c40 mm and is favorable to dividedly arrange the cushion rubber layer at an equal width on both sides with respect to the vertical line VL2.
In this case, it is favorable that a distance d4 from the cord of the innermost carcass ply to the reinforcing rubber layer as measured on the vertical line VL2 is within a range of 0.5xcx9c3.0 mm through the cushion rubber layer.
Moreover, it is preferable to arrange the cushion rubber layer in at least two arrangement positions selected from the above four arrangement positions.
The loss tangent of the cushion rubber layer is within a range of 0.02xcx9c0.10 under test conditions that a temperature is 25xc2x0 C., an initial tensile load is 160 gf, a dynamic strain is 1.0% and a frequency is 52 Hz. Moreover, the method of measuring the loss tangent is as previously mentioned. However, dimensions of a rubber sample applied to the test of loss tangent are 2 mm in thickness, 5 mm in width and 20 mm in length.
In another preferable embodiment of the invention, the cord of the carcass is an organic fiber cord. When the carcass is comprised of two or more plies, at least one ply contains cords of an organic fiber selected from rayon fiber, aromatic polyamide fiber, aliphatic polyamide fiber having a melting point of not lower than 250xc2x0 as measured through differential scanning calorimetry (DSC) and polyester fiber.
And also, in the carcass comprised of 2 or more plies, at least one ply is favorable to be a split ply separated off in a zone beneath the belt. The split ply is the turnup ply or the down ply and is favorable to have a split-off width corresponding to at least 20% of a belt width. Furthermore, at least one ply of the split plies is favorable to contain cords of an organic fiber selected from rayon fiber, aromatic polyamide fiber, aliphatic polyamide fiber having a melting point of not lower than 250xc2x0 C. as measured through DSC and polyester fiber.
As the aliphatic polyamide, nylon-66 or nylon-46 is preferable. As the polyester, polyethylene terephthalate (PET) and polyethylene-2,6-naphthalate (PEN) are preferable.
Further, the invention lies in a pneumatic tire for passenger car comprising a pair of ring-shaped bead cores, a carcass comprised of at least two rubberized plies of radially arranged cords toroidally extending between the bead cores to reinforce a pair of sidewall portions and a tread portion, at least one ply of which plies containing cords of an organic fiber selected from rayon fiber, aromatic polyamide fiber, aliphatic polyamide fiber having a melting point of not lower than 250xc2x0 C. as measured through DSC and polyester fiber, a belt arranged on an outer periphery of the carcass to reinforce the tread portion and comprised of two or more steel cord cross layers, and a reinforcing rubber layer arranged in at least a part of a zone ranging from a position near to the bead portion through the sidewall portion to a shoulder region of the tread portion, characterized in that at least one ply of the carcass is a split ply separated off in a zone beneath the belt.